This section provides tips and techniques for grinding the back curve of a lens made with Trivex material.
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Base Curve Selection Many of the lens manufacturers who work with Trivex lens material produce their single vision lenses with aspheric base curve designs. It’s important to choose a base curve that is within the specified range for the power that is needed. Manufacturers have base curve selection guides to assist you in this step. Some of the newer surfacing software will specify which base curve to select. Going outside the recommended parameters will cause adverse power problems for the eyeglass wearer. This is true of any lens made in any material and is particularly troublesome with aspheric designs. Eyecare professionals who specifically request base curves outside of the recommended range for a specific Rx should be informed that the lenses they receive may not provide the level of aberration control and vision correction they expected from the design they ordered.
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Surfacing Layout Single vision aspheric lenses made from Trivex material have a specific factory-marked base curve center on the blank. This point is the apex (or steepest part) of the curve that defines the lens’ asphericity. This center should coincide with the optical center after the lens has been surfaced.
If it does not, the lens (depending on power) may have undesirable and unpredictable multi-axis cylinder and may need to be remade. Double aspheric lenses have an axis marking line on them that must be properly aligned with the Rx axis. These markings can be prevented from coming off when the surface protector tape is removed if a piece of disappearing cellophane tape is placed over the markings prior to applying the surface protector tape. These markings will be used to verify that the lens’ optical center and axis are aligned with the aspherical apex during finishing layout.
In order to obtain the full optical and cosmetic benefits that lenses made from Trivex material have to offer, it is recommended that layout computing be completed with software that is specifically designed for these lenses. The combination of aspheric design, 1.53 index and minimum thickness variances requires this upgrade to your layout program.
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Surface Taping and Blocking As with any plastic lens material, the base curve of the lenses made from Trivex material should be covered with surface protecting tape prior to blocking and generating. The front of these lenses has a built-in, protective, scratch-resistant layer, but shielding the lens with surface protector tape offers enhanced scratch protection and provides a better bond for the surfacing block. There is also an increased amount of torque created when surfacing this material and the surface protector tape will help provide a good bond with the surfacing block.
Heat is always an enemy of plastic lenses so the less heat lenses made with Trivex material are exposed to, the better they will turn out. That is why a low temperature alloy or wax blocking method is recommended for blocking lenses made from Trivex material. Be sure to allow adequate cooling time after blocking so that the blocking bond is optimal and the lens is not stressed from the heat of the blocking process.
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Generating There are two basic generator types used in today’s optical laboratory. Different procedures are recommended depending on the type of generator being used.
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Manual Sweep Arm and Diamond Quill GeneratorsA combination grit diamond quill meant for both CR-39 monomer and polycarbonate will work well with Trivex material. It is best to run the coolant throughout the manual generating process. Generator operators should use slightly slower sweeps and take less thickness off with each sweep than they normally do with polycarbonate. There is an increased amount of torque produced during the grinding process on lenses made from Trivex material, so slow things down a bit. A very slow final sweep with a minimal thickness cut and with coolant running will give the lens an excellent pre-fining surface. A fluffy, wool-like swarf that can clog the coolant drain is produced when generating lenses made from Trivex material in a diamond generator. Be prepared to clean out the swarf trap after generating each lens.
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Single-Point CNC Dry Milling Generators A two or three flute cutter will work best with Trivex material. Cutters must be sharp in order to produce the best surface quality. If the generator has an optional cribbing setting, use it. This is because edging lenses made from Trivex material causes a lot of torque so if the lens diameter is minimal, there will be less torque. This helps keep lenses on axis during edging. Using the pin bevel setting on a dry mill generator will help reduce wear on your fining and polishing pads and eliminate sharp edges.
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Thickness While lenses made from Trivex material have excellent impact resistance with a 1.0mm center thickness, they may warp over time if placed in frames that exert significant pressure around the lens’ circumference. Because of this, some labs add 0.2mm to 0.3mm of thickness to the lenses. The extra thickness does not alter the final appearance of the glasses noticeably (for example, a 1.0mm edge vs. a 1.3 mm edge) and can help avoid the potential warping problem.
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Fining Some laboratories may choose to use a two-step fining process to surface lenses made from Trivex material. When doing so, use compensated lap tools. These tools have been cut with a slightly steeper curvature to allow for pad stacking. If a second fining pad is stacked on top of the first fining pad, it adds diameter to the lap tool curvature which can cause a noticeable power error in these lenses depending on the lens’ power. Compensated laps correct this problem. Single-use foam lap tools must also be cut to compensate as well.
Fining pads made specifically for lenses made from Trivex material usually have grit ratings of about 300 for the first fining and 1150 for the second fining depending on the pad manufacturer. Some labs find that the pads they use for polycarbonate lenses work well too since there is little difference in the grit sizes of 280 for the first fining and 1000 for the second fining.
Be sure that cylinder machines are adjusted to their fully recommended sweep and stroke settings (on most cylinder machines this will be about 28mm tall for the stroke by 62mm wide for the sweep). If they are not, there may be areas on the lenses that do not fine completely. Use a slightly lower pin head pressure than the usual 20 psi (about 18 psi), a timer setting of 1.5 – 2.0 minutes and the slow speed setting on your cylinder machines for lenses made from Trivex material. These lenses fine out very easily compared to polycarbonate and do not require much excess pressure or time to bring them to an excellent finish.
Do not over work these lenses; it simply is not necessary.
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Polishing Some laboratories have found that polishing pads and polishes designed specifically for Trivex material work very well. Other laboratories have found that they can use pads and polishes recommended for polycarbonate with good results too. Lenses made from Trivex material tend to be very forgiving to the surfacing process as long as the process is not rushed.
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Backside Scratch-Resistant Hard Coating Even though lenses made from Trivex material are considerably more scratch resistant than uncoated polycarbonate lenses, you will still want to add a backside scratch-resistant hard coating. A good quality UV cured tintable coating will improve scratch resistance as well as enhance the tintability of these lenses.
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Anti-Reflective and Mirror CoatingIn general, AR and mirror coating techniques that provide good results for other plastic lens materials work well for lenses made from Trivex material also. As with other lens materials, a good scratch-resistant hard coating is an essential part of the AR and/or mirror coating’s success. Coatings adhere well to lenses made from Trivex material.
Therefore, washing lenses with soap and water is preferable to etching prior to coating. Some coatings can have a detrimental effect on impact resistance.
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Lens Inspection As mentioned earlier, the center markings on single vision aspheric lenses made from Trivex material must align with the optical center after surfacing. Measure the lens thickness using a lens caliper and visually inspect the lens for any flaws before moving on. Lenses that consistently come out with waves or warpage suggest that there was too much heat or pressure applied during surfacing. Check the chiller and coolant waters in the generator and cylinder machines to ensure they are keeping the lenses cool at all times. Double check the pin head pressure on your cylinder machines to be sure they set properly (around 18 psi).
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